The present invention relates to a system and method for controlling supply of fuel to a vehicular internal combustion engine which cuts off the supply of fuel when a predetermined condition is satisfied. The present invention is particularly applicable to vehicles having automataic transmissions.
A main object to carry out cutoff of fuel supply to the vehicular engine is to reduce remarkably fuel consumption and unnecessary exhaust gas. In addition, it is necessary to achieve these items to be reduced efficiently without sacrifice of driveability of the engine.
One of conventional systems described above is exemplified by a U.S. Pat. No. 4,395,984 filed on Sept. 25, 1980 and issued on Aug. 2, 1983.
In the above-identified Patent document, the fuel supply cutoff and resumption are carried out with a throttle valve in a throttle chamber fully closed during deceleration of the vehicle in accordance with a change in an engine speed directly detected by means of a crankshaft rotation speed (crank angle) sensor in order to achieve the above-described main object.
However, the above-described fuel supply control system has a drawback. That is to say, since in a vehicular engine linked with a non-rigid power transmission mechanism constituted by a fluid coupling or torque converter, a conversion rate of torque versus revolution number (engine speed) is different dependent upon a gear position of the non-rigid power transmission mechanism. Therefore, a difference exists between an optimum engine speed at which the fuel supply is resumed after the fuel supply cutoff and the actual engine speed at which the fuel supply is resumed.
As described above, since the conventional system determines to cut off the fuel supply mainly on the basis of the engine speed, the following problems occur.
(I) When a load applied to the engine is large, for example, in a case where such an auxiliary equipment as an air conditioner is operated and therefore a drop in the engine speed is fast, the engine speed decreases even though the fuel supply cutoff is initiated from the engine speed slightly higher than the fuel supply cutoff speed limit and immediately reaches a fuel supply recovery enable speed limit. In this case, the fuel supply cutoff and recovery are carried out in a very short time so that the engine driveability is reduced. (Refer to (C) in FIG. 7)
(II) When the engine is driven from the vehicle via the torque converter, for example, in a case where vehicle travels along a long down hill, the fuel supply cutoff is not carried out even though the engine speed is in the fuel supply cutoff zone since the engine is decelerated from an engine speed below the fuel supply cutoff speed limit N.sub.c. (Refer to (b) of FIG. 7)
(III) Furthermore, since the fuel supply cutoff enable speed limit N.sub.c is fixed, e.g., according to an engine cooling water temperature, it is set to a value slightly higher than an ideal fuel supply cutoff enable speed limit to cope with a characteristic change in each part of the system including the engine and automatic transmission. In this way, in the conventional fuel supply control system, the fuel supply cutoff and resumption are not carried out so as to completely match with the engine operating condition.
The same Applicant has filed a U.S. patent application Ser. No. 719,018 filed on Apr. 2, 1985 (now pending) which discloses the fuel supply system and method to the vehicular internal combustion engine which the above-described drawbacks considerably eliminates and increases the effect of fuel supply cutoff, i.e., saving of fuel consumption.
The Applicant has found that the fuel supply system and method disclosed in the above-identified U.S. Patent Application is still insufficient in that the function of fuel supply cutoff can ideally be enhanced with an appropriate determination of dispersions in characteristics and aging effect of parts in each portion of the fuel supply control system, engine, and power transmission mechanism (hereinafter, these changes are simply referred to as characteristic change).